1. Field of the Invention
The present invention relates to a machining fluid level detection device for wire cut electrical discharge machines.
2. Description of the Related Art
Many wire cut electrical discharge machines are designed to perform machining on a workpiece immersed in an electrically conductive water-based machining fluid stored in a machining tank. The machining fluid is supplied to the machining tank before electrical discharge machining starts. The machining fluid level must be kept at a predetermined height in the machining tank. A float switch is usually used to detect the machining fluid level.
FIG. 2 schematically shows a conventional machining tank using a float switch to detect the machining fluid level. Referring to FIG. 2, the machining tank 1 has a table 2 disposed therein for mounting a workpiece. Machining tank 1 stores machining fluid 3. A ground wire 4 is connected to table 2.
A float 5, which is attached via a float holding metal plate 6 to an upper guide 8 guiding a wire electrode, detects when the machining fluid level reaches a predetermined height in machining tank 1. Reference numeral 7 denotes a signal wire and reference numeral 9 denotes a nozzle provided on the upper guide.
As machining fluid is supplied in the machining tank 1, the machining fluid level rises in proportion to the quantity of machining fluid supplied. When the machining fluid level reaches the predetermined height, float 5 rises and a float switch inside the float operates and outputs through signal wire 7 a machining fluid level detection signal indicating that the machining fluid level has reached the predetermined height. This detection signal stops the supply of machining fluid.
In the machining tank, however, there remains sludge produced by electrical discharge machining. A large amount of sludge accumulates, especially after continuous rough machining. The sludge adheres to moving parts in the float switch and may prevent their operation. In addition, for structural reasons, the signal wire of the float switch is also immersed in the machining fluid, so the signal wire is eventually broken due to electric corrosion by the machining fluid.
To solve the first problem, a known machining fluid level detection device employs two electrodes separated by a predetermined distance (refer to Japanese Patent Application Laid-Open No. S63-123636). In this machining fluid level detection device, a predetermined voltage is applied between two separated electrodes. As both of these electrodes are immersed in machining fluid, current flows between them, so that the machining fluid level can be detected by deciding whether or not the value of the current has reached a certain threshold.
As described above, conventional machining fluid level detection methods using a float switch are unreliable because the float switch may malfunction due to sludge or the signal wire may be broken due to electrical corrosion by the machining fluid. The machining fluid level detection method described in the above Japanese Patent Application Laid-Open No. S63-123636 has problems as well. Some machining fluid used for electrical discharge machining has a high specific resistance of approximately 200,000 Ω/cm. To allow current to flow between two electrodes through such a machining fluid, the electrodes must be as close as possible to each other, or the dimensions of the electrodes must be increased to enlarge the facing areas.
Because the electrodes are placed inside the machining tank, enlarged electrodes would limit the size of the workpiece, or might be hit and damaged by the workpiece. Accordingly, the electrodes must be kept to a reasonable size and the distance between the electrodes must be reduced. In the embodiment described in the above patent document, the distance between the electrodes is 1 mm. Such a narrow space between the electrodes can lead to false detection due to current flowing between the electrodes through a droplet held therebetween by surface tension after the machining fluid is discharged from the machining tank and the machining fluid level is quite low.
The sludge produced by electrical discharge machining stays in the machining tank and accumulates on both the electrodes and the insulators holding them. During long unattended operation, which is common in electrical discharge machining, sludge may accumulate between the electrodes to such an extent that the electrodes are electrically connected to each other. In such a case, current flows between the electrodes even after the machining fluid has been discharged from the machining tank and the machining fluid level is quite low, which may cause false detection.